#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
int main()
{
	int n, flag = 1;
	double sum = 0;
	scanf("%d", &n);

	for (int i = 1; i <= 4 * n - 1; i += 2)
	{
		sum += flag * 1.0 / i;
		flag = -flag;
	}

	printf("%lf", 4 * sum);
	return 0;
}